Plasticity for speech perception changes during childhood and adolescence, resulting in a sensitive period for second-language acquisition. However, the developmental time-course and the underlying cognitive and neural mechanisms of this change are not adequately understood. The proposed research investigates the hypothesis that entrenchment of native language perception is one mechanism of such age-limited learning. Our specific aims are : 1) Examine the influence of age of arrival on perceptual expertise for second-language speech categories. We will study adult native speakers of Japanese who are learning English using a suite of experimental methods including: a) a visual world task that combines psychophysical measures and sensitive eye-tracking measures, b) a task free dishabituation paradigm for fMRI to study the functional anatomy of phonetic categorization and c) a dishabituation paradigm for EEG that matches the timecourse of the behavioral results and d) productions tasks to assess the relationship between perception and production development. 2) Examine the development of phonetic categorization in children and adolescents applying the methods developed above to native English speakers age 6-18. 3) Explore the relationship between the emergence of native-language expertise and the decline in plasticity for second-language speech categories in the data from Aims 1and 2.We hypothesize that the decline in plasticity during the sensitive period is associated with increasing perceptual expertise. Data collected with the same paradigms and stimuli for adult native speakers of Japanese, native English speaking children, adolescents and adults will be the basis for a strong test of this hypothesis. This investigation will provide insights into the typical developmental timecourse of speech perception from childhood to adulthood, which provides a model system in which to study the cognitive and neural mechanisms behind a basic failure of learning. The normative data, paradigms, and mechanistic insights may ultimately benefit studies of atypical development in clinical populations (i.e. dyslexia, language impairments, and learning disabilities).